The present invention relates to a monitoring system of submarine repeaters for digital transmission circuits utilizing an optical fiber.
In an optical transmission circuit, neither light emitter elements nor light receiver elements have satisfactory linear characteristics, and an analog communication system which transmits an analog signal can not be implemented in the optical transmission circuit. Therefore, a digital transmission circuit which transmits a digital signal is presently under study for the application to an optical transmission circuit. Accordingly, none of the monitoring systems which have been utilized in a prior analog submarine cable system is applicable to an optical fiber submarine cable in a digital communication system.
Some of the prominent prior knowledge in the field of monitoring system of onshore digital PCM communication systems are, for instance, "the pulse trio system" disclosed in "the Review of Electrical Communication Laboratories" Vol. 14, No. 1 published by Nippon Telegraph & Telephone Public Corporation, Tokyo, Japan, "the phase detection system" or "AMI violation detecting system" disclosed in "the Review of Electrical Communication Laboratories", Vol. 24, No. 9-10.
However, the above prior arts have the disadvantage that an interstitial cable is necessary in addition to the main communication cable in order to designate the particular repeater and/or transmit a test signal to/from the repeater to be tested. If we try to introduce that interstitial cable system to an intercontinental optical fiber submarine communication system, additional repeaters for interstitial cable are necessary since such an intercontinental submarine cable is too long to transmit without a repeater. That additional repeater for interstitial cable makes the communication system complicated, and the interstitial cable must be as reliable as the main optical communication system. Further, the system is uneconomical since the cable structure is complicated.
On the other hand, it is true that various monitoring systems which do not employ interstitial cables but only the main cable have been proposed. However, none of them are free from the drawbacks itemized below.
(1) Since the system is designed to return a part of the repeater recognition signal itself directly as a test signal, an arbitrary test signal pattern can not be utilized but that test signal pattern is severely restricted.
(2) Since the system is designed to allocate a specific signal to the recognition of each of the repeaters, it is impossible to test all the repeaters with a common test condition, for example with a pattern commonly applicable to all the repeaters, and
(3) Since the system is designed to return a signal by employing an electrical circuit assembled in a repeater, the main communication route is required to have a switch to be connected in series therein (for instance Japanese patent laid-open publication No. 51-99904). However, that electrical switch inserted in the main communication route reduces the reliability of the main communication route considerably.
On the other hand, the Japanese Patent laid-open publication No. 54-133001 discloses another system to solve the above problem. According to the system, a repeater recognition signal which is independent from a repeater test signal, is employed. When receiving the repeater recognition signal, an optical switch forms a return circuit in the repeater during a predetermined duration to allow a repeater test signal to return through the return circuit. The system is, however, involved with the disadvantages below.
(1) Since the duration that the signal is returned is predetermined and is limited to a rather short duration, it is essential to finish a repeater test within that predetermined duration, and so causing the system to be inflexible and inconvenient in operation, and
(2) Adverse effects are assumed for the long term stability of a time constant circuit for defining said predetermined duration.
Another prior proposal returns the signal by an optical switch (no conversion from optical energy to electrical energy is performed), and the repeater test signal doubles as the repeater recognition signal. However, this system does not solve all the problems pointed out in the above, either.